There is a lot wrong in your marriage, if your husband constantly forms a huddle with SD, seemingly shutting you out, and appears to devalue you and your son in favour of her.  You need to take this up with him as a matter of urgency, either have marriage counselling or at the very least a come to Jesus meeting with him so you can tell him how unhappy this is making you.  Don't just suffer in silence - I did that both in my first marriage and in the current one for the first 10+ years, and believe me, this is not the answer.  Make a noise about it. 

I always love DH's "wonderful memories" of when SD was a baby.  He would constantly tell me how much better a baby she was.  Well, at that time he was working 12-15 hour days, and she was at daycare all.day.long.  Why? Because BM needed time for herself (and other men).  He has memories of her constantly spreading poop all over her crib, which is "funny" and "cute".  All her pictures she's in  some contraption in front of a screen.  On top of this she's so smart she never crawled, just went straight to walking, amazing.  My take: it's going to be a better experience when you're not actually having to do the work of watching and entertaining your child.

I agree, it never ends.  My SD58 is a total loser.  She is unemployed, we are subsidizing her shelter expense and she's on disability.  She has trashed every human relationship she ever hsd.  He has her coming weekly to help with yardwork to rationalize the $ he gives her.  Thats because i dont trust her to be in the house doing cleaning.  All i hear is what a good job she does, what a hard worker, her car is so clean, blah blah blah, on and on.  When DHs  dont have anything substantial to praise them for, they find the tiniest crumbs.  It's aggravating.

I'm very grateful that my husband always talks about how happy, sweet, and healthy our daughter is compared to SD who was the total opposite, as was BM's pregnancy.

He's actually afraid to have another kid because he doesn't think he will get this lucky again.

I can completely relate. My SO is the same way about SD11.  I came into her life when she was 7, and at first, I thought he was just a naturally proud father, too, but after 3.5 years of being witness to this UNHEALTHY behavior towards his offspring, I'm over it.  Good healthy fathering can be a huge a turn-on, but this over-the-top praise for even the littlest thing, bragging, pedastilizing (NO other kid can compare to their first offspring from the "golden uterus" BM) and practically licking their kids boots to stay in kids’ favor, is a huge turn off. Add in the almost complete lack of discipline and unwillingness to enforce consequences, and overly indulgent “Disney” parenting, and the whole thing just stinks of unhealthy, unattractive rot!

Seriously, SD11 will do a very normal thing (like put her dishes in the dishwasher after dinner, complete an assigned project for school, or pull some weeds for half an hour) and the lavish praise and adoration she will get from daddy just makes me want to vomit. He practically holds a mini-parade for every little thing.  I pull weeds every day (I manage/tend our veggie garden) and my six-year-old son helps me with chores - weeding, loading/unloading dishwasher, house cleaning, etc, every day, yet she pulls weeds for half an hour one day back in early May and is the best, most hard-working, talented, helpful girl ever? She completes an assignment for school - the same as every other kid in her class - but she’s the brightest, most creative, EVER according to daddy.  She plays a kid-league basketball game - along with a dozen teammates, all of whom had fun and tried hard - but SD11 had the most hussle out there and is definitely the *only* one with pro-athlete potential, don’t you know! (even when she sits on the bench over half the game - I’ve been to many games and have seen this).

I especially relate to what you said about when everyone starts talking at once at the dinner table, who gets his super-focussed attention?  Why, SD11 of course! He will drop everything whenever she enters a room or starts to talk. If he is reading on his phone, he puts it down. If he is eating, he stops eating and looks at her. And hangs on her every word.  With me, if I come into a room and try to talk to him, he looks up from his phone briefly, obviously annoyed, and then continues to scroll or even compose texts while I am trying to talk to him half the time.  And later claims I “never told him this or that.” Yeah, well maybe if you gave me half of the same courtesy you give your daughter, you might remember more that I say! Some of it is important!

Now, SD11 is a really good girl, to be fair.  She does get good grades, she is pretty, and usually empathetic and kind.  She has her moments, like all kids do, but by and large, he has every reason to be proud of her in a healthy way. But I think there’s a huge difference between “being a proud parent,” and this behavior that we see from these divorced dads with their kids, especially daughters, that almost mimics starry-eyed romantic idealization/obsession.

When we first got together and for about the first year, he was very excited about the prospect of having another child. I guess he had wanted another with BM, but she refused.  I, too, always really wanted another.  About a year and a half in, he made some remarks about how he had reconsidered because he already had “everything he could ever want“ and realized he was “completely fulfilled” with his daughter from BM.  This devastated me for so many reasons. Now he has flipped his mind back to *maybe* wanting another with me (how kind of him to throw me that bone...), I am dead opposed to the idea. I feel like I would be bringing “our“ baby into this world already automatically playing second fiddle in his heart and mind to his perfect daughter from first family. And honestly, he is so obsessed with his daughter, I don’t see how he would even have any room in his heart or time in his day to care for and love our baby.  I feel like they certainly wouldn’t be treated equally. The whole idea of an “ours baby” just feels gross to me now.

Yep, it doesn't end. I'm fortunate that my SD40 is a functioning adult, hard worker, and good mom. But DH still gets gushy when he talks about her. I get loving your kids -- but this worshiping them is just nauseating.

I have address it with him, explaining how his behavior affects me. "DH, do you realize you exclude me from conversations even when I try to participate? DH, do you realize you hold SD's hand and walk in front of me?" That sort of thing. And that works for an SD who is an adult and who we see only a few times a year.

Living with it every day, and with a mutual child, too, would drive me insane. I'd have to confront him.

This statement right here from Tessa LeAnn:

"But I think there’s a huge difference between “being a proud parent,” and this behavior that we see from these divorced dads with their kids, especially daughters, that almost mimics starry-eyed romantic idealization/obsession."

Where does this behavior from these dad's stem from? What's the psychology behind it? In my case, the ex had 2 kids but clear as day favored his daughter over his son and was often mean to his own son. If he was equally enamored with both kids I would write it off as a spineless Disney dad and be done. But it was the daughter that was on a pedestal and she was the worst behaved/least sanitary of the two kids which made the obsession with her even more confusing. It, at times, felt incestual the way my ex was obsessed with her. To be clear, I never suspected incest. It was just a bizarre, unnatural pining for his daughter that felt like puppy love rather than a father/daughter relationship. 

Wow, I'm so sorry you have to put up with your empty-headed SO comparing your son to his brat and being belittled by them. I can only imagine how awful it feels. DH only began to compare our baby to stepbrat before I put a stop to it. Now all I hear from him is how much of an easy baby our son is, how happy and calm he is. How smart and good he is "compared to other kids" I know he refers to SD who according to him has always been a clingy crybaby and quite slow. BM never included him on anything SD or pregnancy related so he's living all these first experiences with me and I love seeing how happy he is doing it. 

I think it is time for webcams and recorders.  Everywhere and all of the time.  Then edit the highlights reel for DH every month or so showing his doting on SD, ignoring DS and marginalizing you.

I would say that it is time to shut DH down completely with his constant blather about the superiority of SD.   Inform him that if he does not start stepping up as a husband for you and a father for your son that you will find someone who will and he will have another couple of decades of CS to pay.

Direct, and to the point.

Good luck.